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Archive for the ‘Diseases Caused by Fungi’ Category

Fusarium Dry Rot and Wilt

Thursday, September 10th, 2009

(Fusarium spp.)

Different Fusarium species distributed worldwide cause various problems. Warm temperatures favours this disease.

Symptoms of Fusarium Dry Rot and Wilt

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Dry rot is one of the most serious potatoes storage problems. Tubers initially have dark, slightly sunken lesions that later expand superficially, leaving internal cavities that may contain different colour mycelia, depending on the species involved.
The margin of the rot is clearly defined.
Concentric rings appear on the potatoes tubers surface and external mycelium is evident.
Potatoes tuber dries and hardens.
Under humid conditions, secondary soft rot develops. Infection originates in surface fusaruim-dry- rot-wiltwounds during harvest and handling.
It can be reduced by initial curing at about 15°C and 95% relative humidity to promote wound suberization, prior to cold storage.
Improperly suberized cut seed decays under adverse soil conditions.
Potato plants may fail to emerge, or be weak and subsequently wilt and die.
Fusarium wilt fungi are soil-borne.
Symptoms are yellowing of lower leaves, chlorotic mottle of upper leaves, and subsequent wilting.
Vascular tissues of stems and tubers become discolored.
Potatoes tubers show several types of internal and external discoloration such as brown sunken necrosis at the stolon attachment or eyes, and circular brown rotted areas.
Warm weather enhances wilt. Some Fusarium strains become systemic and are seed transmitted.

Management

Use disease-free potatoes seed, good water management, and crop rotation. Treat cut seed with chemical protectants.


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Stem Canker and Black Scurf

Thursday, September 10th, 2009

(Rhizoctonia solani)

The fungus causing stem canker and black scurf is present in nearly all soils because it has a wide host range, survives in plant debris, and as sclerotia is easily disseminated on tubers.
It grows over a wide range of temperatures. It causes considerable damage to emerging sprouts when conditions do not favor rapid emergence, such as cold and wet soil.

Symptoms of stem canker and black scurf

stem-canker-black-scurfLesions on sprout tips cause delayed emergence or failure to emerge.

Slightly sunken brown cankers of variable size and shape affect stolons and stems at or below the soil line.
Cankers may girdle stems and result in aerial tuber formation, plant wilt, and death. Girdled stolons may fail to produce tubers.
Hard, dark brown or black sclerotia (fungus-resting bodies) of irregular size and shape form on the tuber surface.
A white mycelial mat may develop on the stem base, but does little harm to the plant.

Management

Because sclerotia are long-lived in the soil, while growing potatoes only long rotations with cereals and grasses reduce disease incidence.
Shallow planting of well-sprouted tubers reduces the exposure time of sprouts in the soil.
This potatoes disease can be reduced by applications of soil fungicides such as PCNB (pentachloronitrobenzene) mixed into the planting band of soil.

Potatoes seed tuber treatment effectively reduces seed-borne inoculum when soils are not heavily infested.

Trichoderma and binucleate Rhizoctonia used as biological control agents reduce severity.

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Black Rot

Thursday, September 10th, 2009

(Rosellinia sp.)

Black rot, in higher tropical altitudes, may cause heavy potato yield losses in moist soils rich in organic matter (recently cleared land) and may affect other crop and weed hosts.

Symptoms of Black Rot

Black-rotDiseased potato plants are stunted and wilt. Underground organs develop a black rot partially covered by matted strands of grayish white mycelium.

The blackened potatoes tubers in cross section show a band of striate projections penetrating into the flesh. Often, only isolated potato plants are affected or the disease occurs in patches in the field. Only during lack of crop rotation while growing potatoes does the disease become a major problem.

Management

Removal or burning of brush and trees when land is cleared and subsequent crop rotation are recommended control measures.

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